In the chemical industry the process of obtaining new substances or compounds generally follows a basic development route starting in a small-scale volume where many different substances or compounds may be evaluated. Further on in the development route where a specific substance or compound should be tested, e.g. in the case of pharmaceutical substances where many test procedures have to be performed or when developing new materials for the semiconductor industry, much larger volumes are needed than those available in the initial small scale stages.
A particular requirement of systems for performing these tasks is that most processes takes place under a pressure which may be ten times normal atmospheric pressure.
Thus, there exists a general demand in the chemical industry to be able to obtain larger volumes of a substance or compound, under such processing conditions. This should however not be regarded as limiting the scope of the invention. The present invention is applicable for volumes from 10 mL and larger.
There is very often a need to very rapidly heat up the load volume, a typical design criterion being 5 K/sec. Supposing 50 mL of a typical liquid with a heat capacity of 2 J/mL, K is the power requirement and becomes 500 W. To feed this high power into a small cavity may be problematic, in particular with the need for a pressure sealed microwave feedthrough, and the need for a reasonably even heating of the load is a further difficulty.
Microwave assisted chemistry has been used for many years. However, the apparatuses and methods have to a great extent been based upon conventional domestic microwave ovens. Domestic microwave ovens have a multimode cavity and the energy is applied at a fixed frequency at 2450 MHz; the available microwave power is up to 1 kW but the fact that such ovens are not designed for loads of this kind result in a typical microwave efficiency way below 50%. The use of single mode cavities have also been reported, see e.g. U.S. Pat. No. 5,393,492 and U.S. Pat. No. 4,681,740.
Recent developments have led towards apparatuses comprising a microwave generator, a separate applicator for holding the load (or sample) to be treated, and a waveguide leading the generated microwave energy from the generator and coupling it into the applicator. Even if the system consists of a TE10 waveguide using a 2450 MHz to which a magnetron generator is connected in one end and the sample container is in the other end, there is a need for a matching device in the form of at least a metal post or iris between the generator and load, in order to achieve a reasonable efficiency.
When coupling electromagnetic radiation such as microwaves from a source to an applicator, it is important to match the transmission line impedance to the applicator impedance (with load) in order to achieve a good transfer of power. It is of particular importance that a range of liquids over a range of temperatures can be used. However, the dielectric properties of the load then vary considerably and may influence drastically upon the impedance of the applicator, as well as its electrical size. Thus, an impedance mismatch between the source and the applicator will often occur and the coupling and thereby the heating process becomes less efficient and difficult to predict.
Below follows a short background description of different transmission modes used in a microwave applicator.
Consider a hollow waveguide with a given cross section that is uniform throughout its entire length. According to the known theory, a discrete number of two types of modes are then possible within a limited frequency range-the transverse electric (TE) mode and the transverse magnetic (TM) mode. TE modes have only E field components transverse (that is perpendicular) to the direction of propagation, whereas the H field has both transversal and longitudinal components. TM modes have only H field components transverse (that is perpendicular) to the direction of propagation, whereas the E field may have both transversal and longitudinal components.
One of the most important characteristics of TE and TM modes is that there is a cutoff wavelength for each mode of transmission. If the free-space wavelength is longer than the cutoff value, that particular mode cannot exist in a long waveguide. For any given waveguide, the mode that has the longest cutoff wavelength is known as the dominant mode. The particular mode is given in index form and, as an example, in a rectangular waveguide the TE10 mode is dominant.
A transverse magnetic type mode with indexing using the nomenclature for circularly polarized cylindrical resonators is TMmnp where m is the circumferential direction, n is the radial direction, and p is the axial direction of propagation.
In U.S. Pat. No. 5,834,744 a tubular microwave applicator for applying microwaves to a load having a generally circular cross-section is described. The applicator supports a dominant TM120 mode and the load is aligned with a central axis in the applicator. The applicator has an airfilled microwave cavity fed by a pair of slot apertures coupling microwave energy from a waveguide feed system connected to a source of microwave energy. The waveguide supplies a symmetrical rectangular TE10 mode split into waveguide arms with slot apertures sized and positioned to only excite the TM1 mode type in the applicator cavity.
The applicator disclosed in U.S. Pat. No. 5,834,744 is supplied with microwave energy from the waveguide feed system having cavity feeding ports in the applicator periphery, i.e. from a radial direction.
This known applicator has a limitation with regard to the ability to achieve an efficient pressure seal of the applicator due to the radial direction of the feed system. Thus, arranging a proper pressure seal may be rather expensive and technically difficult.
An applicator geometry suitable for treating a load under pressure is described in a Technical Note [Matusiewicz, Development of a High Pressure/Temperature Focused Microwave Heated Teflon Bomb for Sample Preparation, Analytical chemistry Vol. 66, No 5, 1994]. A cylindrical steel vessel is lined with a ceramic material on the inside, and the load is located in a frustum conical container inside. The microwaves are fed into the structure by a coaxial line from below, connected to an internal coupling antenna system in the ceramic. The top of the vessel is closed by a compression plate and a lug.
The antenna system has a quite complicated shape that makes it problematic and expensive both to manufacture and to control with regard to the need for tight contact with the ceramic material. Furthermore, the actual shape and design of the antenna system are given only in a schematic figure. From that figure it is, however, possible to conclude that the antenna system is either in a rotationally symmetric cup shape, or in wire or thin plate U shape, in both cases with a symmetric feed by the coaxial center conductor from below. It is indeed very unclear if the described system is capable of being used with the high microwave powers envisaged for the system of the present invention; as a matter of fact the description gives only one example, at 90 W. Not only is the manufacturing process of the antenna system problematic, but it can also be unambiguously shown by microwave modeling using the data in the schematic figure that either a rotationally symmetric cylindrical TM0 mode or (in the case of the complicated U-shaped antenna) more complicated degenerated modes having no useful polarization or other property which allows refinement or optimization of the design for anything but impedance matching—the feed and antenna system is a single solution which must be used as is and cannot be complemented by for example a second orthogonal system.
The object of the present invention is to achieve a microwave heating apparatus adapted to heat load volumes of 10 mL or more, which can be used with microwave power levels exceeding 1 kW if desired, and that may also be pressure sealed in a cost efficient way.